Compositions and methods for planarizing or polishing the surface of a substrate are well known in the art. Polishing compositions (also known as polishing slurries) typically contain an abrasive material in an aqueous solution and are applied to a surface by contacting the surface with a polishing pad saturated with the slurry composition. Typical abrasive materials include silicon dioxide, cerium oxide, aluminum oxide, zirconium oxide, and tin oxide. U.S. Pat. No. 5,527,423, for example, describes a method for chemically-mechanically polishing a metal layer by contacting the surface with a polishing slurry comprising high purity fine metal oxide particles in an aqueous medium. Alternatively, the abrasive material may be incorporated into the polishing pad. U.S. Pat. No. 5,489,233 discloses the use of polishing pads having a surface texture or pattern, and U.S. Pat. No. 5,958,794 discloses a fixed abrasive polishing pad.
Conventional polishing systems and polishing methods sometimes are not satisfactory at planarizing semiconductor wafers. In particular, polishing compositions and polishing pads can have less than desirable polishing rates, and their use in the chemical-mechanical polishing of semiconductor surfaces can introduce defects into the resulting integrated circuits. Because the yield from a semiconductor wafer is associated with the ability to planarize layers during fabrication without introducing surface defects, it is important to use an effective and efficient polishing system. Manufacturing processes for integrated circuit wafers rely on polishing steps to remove material and planarize the wafer before additional layers are added. The difficulty in creating an effective polishing system for semiconductor wafers stems from the complexity of the semiconductor wafer. Processed semiconductor wafers typically include a plurality of transistors or other semiconductor devices formed over multiple layers together with complex, sometimes multi-metal, conductor patterns used to interconnect the semiconductor devices. Accordingly, polishing steps must be applied to surfaces presenting patterns formed of different materials.
Various metals and metal alloys have been used to form electrical connections between devices, including titanium, titanium nitride, aluminum-copper, aluminum-silicon, copper, tungsten, platinum, platinum-tungsten, platinum-tin, ruthenium, and combinations thereof. Because of their desirable qualities such as inertness, noble metals, including ruthenium, iridium, and platinum, are finding increasing use in memory devices and metal gates. Noble metals, however, present a particular processing challenge at the polishing steps, however. They are mechanically harder and more chemically resistant than many of the other integrated circuit conductor metals. As the noble metals are often used in patterns with other softer and more readily abradable materials, including dielectric materials such as silicon dioxide, problems of selectivity in polishing of the noble metals versus over-polishing of the dielectric materials must be addressed.
Efforts at developing slurries for noble metal polishing are reflected in the patent literature. For example, U.S. Pat. No. 5,691,219 is directed to a semiconductor memory device with a noble metal conductive layer and a polishing composition comprising a halo-compound for polishing the noble metal. U.S. Pat. No. 6,290,736 discloses a chemically active polishing composition for noble metals comprising an abrasive and a halogen in basic aqueous solution. International Application Publication No. WO 00/77107 A1 discloses a polishing composition for noble metals (e.g., Ru, Rh, Pd, Os, Ir, Pt) comprising an abrasive, a liquid carrier, an oxidizer, and a polishing additive that can include EDTA, nitrogen-containing macrocycles (e.g., tetraazacyclotetradecanes), crown ethers, halides, cyanides, citric acid, phosphines, and phosphonates. With regard to polishing-slurries suggested for polishing other conductor patterns, e.g. tantalum, International Application Publication No. WO 03/072683 A1 is directed to a chemical-mechanical polishing slurry having particles surrounded by a shell formed of an adsorption additive.
A need remains for improved, cost-effective polishing systems and polishing methods that will exhibit desirable planarization efficiency, material selectivity, and defect abatement. The invention provides such a method. These and other advantages of the invention, as well as additional inventive features, will be apparent from the description of the invention provided herein.